What Scaffold Planking Has to Carry and How It Is Checked

Why this matters

Planking is checked from the ground, from an adjacent verified level, or from the bay below with the deck above supported and unloaded - never by walking out onto the board you are trying to evaluate. If the only way to test a plank is to stand on it, the plank is out of service until the competent person, who under 29 CFR 1926.32(f) has authority to stop the work, has cleared it another way.

A plank is the one scaffold component a crew feels qualified to judge by eye, because it is lumber and everyone has handled lumber. The claim of this card is that eye judgment is looking at the wrong noun. A plank's capacity is a statement about a span and a duty class, not a statement about a board. The same physically identical plank is comfortably adequate across one bay and unsafe across another, and no amount of looking at it tells you which case you are in, because the number that changed is on the scaffold, not on the wood.

The four inputs that fix a plank's capacity

Duty class. Scaffold platforms are classified by the uniform load they are intended to carry. Light duty, medium duty and heavy duty are the standard classifications, running at 25, 50 and 75 pounds per square foot respectively in the duty classification scheme used by Appendix A to 29 CFR 1926 Subpart L and by ANSI/ASSP A10.8 in whatever edition your employer's programme or your contract has adopted. The class is decided by the work, before anyone picks up a board.

Span. The distance from the centre of one support to the centre of the next. This is the variable that does the damage, because it is the one that changes bay to bay while the plank stays the same.

Plank type and grade. Solid-sawn scaffold plank is a graded product carrying a grade stamp from a grading agency, and its permissible spans come from the grading rules and from Appendix A. Fabricated planks and decks carry their own manufacturer's rating instead. A dimension board off the general lumber pile is not scaffold plank no matter how sound it looks, and there is no inspection that converts one into the other.

Actual thickness. Appendix A distinguishes full-thickness undressed lumber from nominal-thickness lumber and gives them different permissible spans, which is the standard telling you outright that a board's dressed dimensions are load-bearing information.

Permissible span falls as duty class rises, and it falls further for nominal-thickness rather than full-thickness lumber. Read the actual feet out of Appendix A or the manufacturer's rating for the exact plank in your hands. That table owns the number, and reciting a span from memory across two different plank types is how a crew gets it wrong in the direction that fails.

The geometry at each end

   overhang span, center of support overhang
  |<------>|<---- to center of support --->|<------>|
  +========+===============================+========+
           ^                               ^
        support support

29 CFR 1926.451(b) sets the rules that diagram encodes. Each end of a platform extends over the centreline of its support by at least 6 inches unless the plank is cleated or otherwise restrained from movement, and does not extend more than 12 inches beyond the support unless the end is guarded or the platform is designed and installed for the longer overhang. The platform is fully planked between the front uprights and the guardrail supports, with gaps held to a minimum.

Both limits exist for different reasons and it is worth knowing which is which. The minimum protects against the plank sliding off its support when someone walks the end of it and the board shifts. The maximum protects against the plank being used as a cantilever by someone who does not realise the far end will lift.

That geometry also fixes the plank length you need, and people routinely get it a foot short. A 7-foot span, centre of support to centre of support, with the minimum 6 inches of bearing at each end, needs a plank at least 8 feet long. Round the required length up.

The build sheet, filled in

Here is a real platform resolved field by field, on a light duty facade job. Every qualifier from above gets a line, and the arithmetic is carried through.

Field 1, duty class. Two techs and a bundle of material at a time, no stockpiling on the deck. Light duty, 25 pounds per square foot.

Field 2, span. Bay is 7 feet, support centre to support centre.

Field 3, plank type and grade. Solid-sawn scaffold plank with a legible grade stamp, nominal 2 by 10, actual width 9.25 inches. Confirm the permissible span for this plank type at 25 pounds per square foot from Appendix A or the grading rules, for the actual thickness in hand and not the nominal one.

Field 4, platform width. Four planks across: 4 times 9.25 inches is 37 inches, which is 3.083 feet. 29 CFR 1926.451(b) sets a minimum platform width of 18 inches for most scaffolds, so 37 inches clears it with room.

Field 5, deck area. 7 feet by 3.083 feet is 21.6 square feet, taken to the nearest tenth.

Field 6, maximum intended load at this class. 25 pounds per square foot times 21.6 square feet is 540 pounds. That is the entire budget for this bay: people, tools, and material.

Field 7, what the crew intends to put on it. Two workers at 250 pounds each including clothing and hand tools - an illustrative round figure, and one to round up rather than down if your crew carries heavier belts - is 500 pounds. Add a 100-pound bundle of material and the total is 600 pounds.

Field 8, the comparison. 600 against a 540-pound budget. Over by 60 pounds, which is 11 percent of the budget. Not close, not marginal, over.

Field 9, what does not fix it. Do not multiply the 540 by the factor of 4 in 29 CFR 1926.451(a)(1) and go looking for a plank rated at 2,160 pounds. That factor is already inside the published allowable and inside the permissible spans in Appendix A. This is a re-basing point, not an addition point: the tabulated span you read has the design factor in it, and applying the factor again is applying a correction that has already been made.

Field 10, what does fix it. Three options, in the order a shop should reach for them. One worker plus the bundle is 350 pounds, comfortably inside 540. Or the material lands on a separate bay rather than the working bay. Or the platform is built to a higher duty class, which means a shorter permissible span for the same plank, which means an extra ledger and a new bay layout - a design decision that goes back to the scaffold's basis, not a decision made on the deck.

Field 11, the deflection check. 29 CFR 1926.451(f) limits platform deflection under load to one sixtieth of the span. At a 7-foot span that is 84 inches divided by 60, which is 1.4 inches. A plank that sags more than 1.4 inches under its intended load has failed the standard's own criterion even though it did not break and even though it will spring back. Deflection is the field-measurable proxy for a plank that is over its span, and it is the only one available without test equipment.

Field 12, plank length and restraint. 7-foot span plus 6 inches of bearing each end gives 8 feet minimum, and the overhang at each end is between 6 and 12 inches. Where the layout forces a longer overhang, the end is guarded or the plank is cleated and the arrangement goes back to the design.

What a plank inspection can and cannot see

The visual check is real and it is not the capacity check. Look for and reject: a plank with no legible grade stamp, splits running through the depth at or near a support, deep notches or drilled holes, paint or opaque coating that hides the grain and therefore hides the defect, saturation and delamination on a fabricated deck, and end splits that have grown since last shift.

None of that answers the span question. A perfect plank across too long a bay is the failure this card exists to prevent, and it presents as a board that looked fine to everyone who walked past it.

What flips the answer. A fabricated aluminium or laminated deck unit replaces the whole Appendix A route with the manufacturer's rating for that unit, at that span, in that duty class, and that rating governs. Where a plank spans supports at unequal heights, or bears on a support that is itself a modified part of the scaffold, the span table stops describing the situation and the question returns to the qualified person who owns the scaffold's design. And any plank stored outdoors through a season gets re-inspected on grade stamp legibility and on end splitting before it is decked, because both degrade in storage rather than in service.

How to verify you got this right

Standing at the base, before the deck is loaded:

• State the duty class out loud and confirm it matches the work actually planned for the shift, not the work planned when the scaffold went up.
• Measure one bay centre to centre and check that span against the table for the plank type in hand.
• Multiply span by planked width, then by the duty class, and compare against people plus tools plus material. Do this once per platform, not once per job.
• Sight along each plank end for the 6 to 12 inch overhang, and check the deck for gaps.
• Load the platform to its intended load with people standing at mid-span and sight the sag against the guardrail line or a string. More than one sixtieth of the span is a stop.

The failure mode is specific and repeatable: a crew builds a deck correctly on a 5-foot bay, moves the scaffold to a corner where the bay opens to 8 feet, and re-uses the same planks because the planks are fine. The planks are fine. The platform is not, and nothing about the boards has changed to tell them so.

References

• 29 CFR 1926 Subpart L, Scaffolds (construction), including 1926.451(a)(1) on capacity and the 4-to-1 factor, 1926.451(b) on platform construction, bearing and overhang, and 1926.451(f) on deflection and on inspection by a competent person before each work shift.
• Appendix A to 29 CFR 1926 Subpart L, which tabulates duty classifications against maximum permissible spans and distinguishes full-thickness undressed lumber from nominal-thickness lumber.
• ANSI/ASSP A10.8, in the edition adopted by your employer's programme or your contract, which is the consensus standard behind the duty classification scheme and binds through that adoption rather than on its own.
• Grade stamps and grading agency rules for solid-sawn scaffold plank, and the manufacturer's rating for fabricated planks and decks, which own the permissible span for the specific product.
• See related: universal-what-a-scaffold-has-to-have-before-anyone-stands-on-it for the rest of the pre-use condition set.